It is known that polymaleimides can be utilized for the preparation of various polyaddition and polymerization products. Particular emphasis has been placed on bis-maleimide materials which exhibit thermal stability and good mechanical properties and, thus, are being more frequently utilized in high performance composite applications, such as electronic circuit board applications.
The currently used bis-maleimide systems include alkenyl phenols as coreactants. U.S. Pat. Nos. 4,100,140; 4,127,615; 4,130,600 and 4,131,632 are examples of patents which disclose crosslinked polymers resulting from the reaction of polymaleimides with alkenyl phenols or alkenyl phenols ethers optionally in the presence of epoxy resins.
A standard polyimide pre-preg for use in electronic circuit board applications is based on bismaleimidodiphenyl methane and diallylbisphenol A. The product normally must be stored at refrigerated temperatures due to the instability and advancement of the resin system at higher temperatures and does not meet the Underwriters Laboratory VO requirement for commercial flame retardancy.
It is a primary object of the present invention to provide a flame retardant polyimide resin system which substantially eliminates the disadvantages encountered with prior art materials.
It is a further object to provide such a flame retardant polyimide resin system without any substantial adverse impact on the thermal and mechanical properties thereof.
It is a still further object to provide a flame retardant polyimide resin system with improved solubility which forms a stable polyimide resin solution.
Various other objects and advantages of this invention will become apparent from the following descriptive material.
It was discovered that the addition and reaction of the tribromophenyl-maleimide into the polyimide resin system in accordance with the present invention provides flame retardancy while not imparting the processing parameters or the thermal, mechanical, chemical or electrical properties of the products manufactured therefrom. In particular, the flame retardant polyimide resin system of the present invention provides pre-pregs with substantially improved storage stability. Thus, the flame retardant polyimide resin composition in accordance with the present invention provides manufactured pre-pregs which can be stored at room temperature without adverse effects to processing conditions.
Surprisingly, the addition of the tribromophenyl-maleimide does not adversely effect the processing characteristics of the polyimide resin system. For example, the extended pot-life of the resin system in accordance with the present invention is not effected which provides improved processability for the manufactured products, especially pre-pregs; the residual volatile content in the pre-preg is unchaged, and the shortened curing rate for the resin system is not effected. Accordingly, the flame retardant polyimide resin composition in accordance with the present invention provides improved coating and processing conditions for higher quality material.
The flame retardant resin systems have application in a broad range of end uses such as in printed circuit boards, castings, composites, molding compounds, adhesives and coatings.